Triaryl thiophosphates are known plasticizing and flame retardant agents for polymers. However, symmetrical triaryl thiophosphates, e.g., triphenyl phosphate, are crystalline solids. This detracts from their use as plasticizers as they have a tendency to crystallize and bloom to the surface of the plasticized composition. As solids, these compounds are difficult to process and handle. They also have a disagreeable H.sub.2 S odor.
Mixed triaryl thiophosphates, such as di-phenyl mono-p-tertiarybutylphenyl thiophosphate, as disclosed in U.S. Pat. No. 2,250,049 are liquid. These mixed triaryl thiophosphates are useful as plasticizing agents and fire retardants for polymers. However, these agents are not substantially odor free.
The mixed triaryl thiophosphates as disclosed in U.S. Pat. No. 2,250,049 are prepared by reacting a phosphorus trihalide with two molecular equivalents of phenol and thereafter reacting the intermediate product with one molecular equivalent of a phenol having nine or more carbon atoms. The reverse sequence is also disclosed as operative. The reaction product consists mainly of the desired mixed triaryl phosphite but may contain minor amounts of other mixed triaryl phosphites as well as symmetrical triaryl phosphites. The thiophosphate is then formed by reaction of the phosphite with sulfur. The product can be used directly or vacuum distilled. The sulfurization step can be eliminated by the use of thiophosphoryl chloride.
In the preparation of these materials, fractional distillation is used to purify the product. However, distillation results in decomposition of the product resulting in the formation of disagreeable odors. It would be desirable to prepare purified liquid triaryl thiophosphates without the need for distillation and the resulting odor problem.
It is well known that the percent phosphorus added as part of a flame retardant is correlatable to the effectiveness of the flame retardant. Also, the expense of an additive can be reflected in the number and complexity of processing steps needed to prepare the additive as well as the amount which must be added to achieve a certain degree of effectiveness.